There was already an article posted here on how to check BSNL broadband online. The procedure explained earlier has changed and now you can check BSNL broadband usage using Online portal as explained below.
In the BSNL SelfCare Portal page, click the View Your PostPaid Usages link.
In the Usage Details page, select the service name as Broadband then pick a drop down value for Service User Id, Usage Month and click the Search button.
This would display the total usage details for the month along with the usage details for each day under the selected month.